The present invention relates to an apparatus and a method for attaining, maintaining constant and limiting a predetermined operating temperature, preferably for a gas-heated liquid petroleum gas evaporator. According to DIN standard 30 696 for liquid petroleum gas evaporators, Section 3.5.3, evaporators must be so equipped and controlled that an exiting gas temperature of between 40.degree. C. and 80.degree. C. can be maintained. All that the DIN standard says about the temperatures arising in the interior of the evaporator is that a temperature limiter must be present, which either limits the temperature of a heat transfer medium to a maximum of 100.degree. C., or prevents the exiting gas temperature from exceeding 90.degree. C. Electrically heated evaporators are sometimes operated without the interposition of a heat transfer medium, and at the transitions between the heating device and the liquid petroleum gas (LPG) they often reach temperatures far above 100.degree. C. In the American-made ALGAS and MITCHELL gas-heated evaporators, LPG-gilled pressure vessels are heated with a direct flame. Here, temperatures of up to approximately 500.degree. C. can arise, while the exiting gas temperatures are at approximately 120.degree. C. For these and other reasons, these apparatuses are not in accordance with DIN standard 30 696 and are not allowed in Europe.
According to Heinz Cordes in his work Propan-Ratgeber (Propane Adviser) published by the Strobel Verlag in Arnsberg, Federal Republic of Germany, oily deposits begin to be produced in the evaporation of LPG at temperatures of approximately 80.degree. C. or above, which then soil the ensuing fixtures such as regulators, diaphragms, valves and so forth and make them sticky. At still higher temperatures, carbonization of the fuel sets in. For this reason, sediment traps and filter devices are typically provided following the evaporators.
German Pat. No. 3 307 520 describes an evaporator that operates with gas-heated coils--without a pressure vessel. According to European Patent Office Publication No. 0 126 852 A2 (Patent Journal 84/49 dated Jan. 14, 1985), the evaporator according to the above patent is additionally equipped with a means of electric preheating, with which the evaporator is initially preheated to the desired operating temperature without drawing fuel. In practice, it has now been found that the current required for the electric preheating is not always available, for instance in road construction machines and for heating points in railroad switches. Furthermore, the temperature control is too sluggish, and depending on the manner of operation and the fuel drawn at a given time, temperature fluctuations occur which may also be higher than the above-described limit. If current is cut off, which may be necessary on weekends to enable repair work to be done, the evaporator and the heating system connected to it shut down as well, which can cause considerable disruptions in operation.